http://www.sciencedirect.com/science/article/pii/S0264817220300131 Numerous marine hydrocarbon seeps have been discovered in the past three decades, the majority of which are dominated by methane-rich fluids. However, an increasing number of modern oil seeps and a few ancient oil- seep deposits have been recognized in recent years. Oil seepage exerts significant control on the composition of the seep-dwelling fauna and may have impacted the marine carbon cycle through geological time to a greater extent than previously recognized. Yet, distinguishing oil-seep from methane-seep deposits is difficult in cases 13 where δ C values are higher than approximately -30‰ due to mixing of different carbon sources. Here, we carb present a comparative study of authigenic carbonates from oil-dominated (site GC232) and methane-dominated (site GC852) seep environments of the northern Gulf of Mexico, aiming to determine the geochemical characteristics of the two types of seep carbonates. We analyzed (1) major and trace element compositions of 13 carbonates, (2) total organic carbon (TOC), total nit rogen (TN) and carbon isotope (δ C ) of residue after TOC 34 decalcification, (3) sulfur isotope signatures of chromium reducible sulfur (CRS, δ S ) and residue after CRS CRS 34 extraction (δ S ), as well as (4) sulfur contents (TOS) of residue after CRS extraction. Carbonates from the TOS 34 studied oil seep are dominated by aragonite and exhibit lower δ S values, suggesting carbonate precipitation CRS close to the sediment surface. In addition, oil-seep carbonates are characterized by higher TOC and TOS 13 contents and higher TOC/TN ratios, as well as less negative δ C values compared to methane-seep TOC carbonates, probably reflecting a contribution of residual crude oil enclosed in oil-seep carbonates. Very low 13 δ C values (as low as -68.7‰, VPDB) and low TOC/TN ratios of methane -seep carbonates indicate that the TOC enclosed organic matter is derived mainly from the biomass of methanotrophic biota. This study presents new geochemical data that will allow the discrimination of oil-seep from methane-seep deposits. Although some of the geochemical patterns are likely to be affected by late diagenesis, if applied with caution, such patterns can be used to discern the two end-member types of seepage - oil seeps and methane seeps - in the geological record.

Šupraha, L., Henderiks, J., 2020. A 15 million-year long record of phenotypic evolution in the heavily calcified Helicosphaera coccolithophore and its biogeochemical implications. Biogeosciences Discussions 2020, 1-26.

https://www.biogeosciences-discuss.net/bg-2019-472/ The biogeochemical performance of coccolithophores is defined by their overall abundance in the oceans, but also by a wide range in cell size, degree of calcification and carbon production rates between different species. Species' sensitivity to environmental forcing has been suggested to relate to their cellular PIC : POC ratio and other physiological constraints. Understanding both the short and longer-term adaptive strategies of different coccolithophore lineages, and how these in turn shape the biogeochemical role of the group, is therefore crucial for modeling the ongoing changes in the global carbon cycle. Here we present data on the phenotypic evolution Helicosphaera of a large and heavily-calcified genus (order Zygodiscales) over the past 15 million years (Ma), at two deep-sea drill sites from the tropical Indian Ocean and temperate South Atlantic. The modern species Helicosphaera carteri , which displays eco-physiological adaptations in modern strains, was used to benchmark the use of its coccolith morphology as a physiological proxy in the fossil record. Our results show that, on the single-genotype level, coccolith morphology has no correlation with physiological H. carteri traits in . However, significant correlations of coccolith morphometric parameters with cell size and physiological rates do emerge once multiple genotypes or closely related lineages are pooled together. Using Helicosphaera this insight, we interpret the phenotypic evolution in as a global, resource limitation-driven selection for smaller cells, which appears to be a common adaptive trait among different coccolithophore lineages, from the warm and high-CO world of the middle Miocene to the cooler and low-CO conditions of 2 2 Helicosphaera the Pleistocene. However, despite a significant decrease in mean size, kept relatively stable PIC : POC (as inferred from the coccolith aspect ratio) and thus highly conservative biogeochemical output on the cellular level. We argue that this supports its status as an obligate calcifier, like other large and heavily- Calcidiscus Coccolithus calcified genera such as and , and that other adaptive strategies, beyond size-adaptation, must support the persistent, albeit less abundant, occurrence of these taxa. This is in stark contrast with the Emiliania Gephyrocapsa ancestral lineage of and , which not only decreased in mean size but also displayed much higher phenotypic plasticity in degree of calcification while becoming globally more dominant in plankton communities.